Monte Carlo simulations for in-beam PET monitoring in hadron-therapy

Monte Carlo simulations for in-beam PET in hadrontherapy. INTRODUCTION: Hadrontherapy is a promising therapy in oncology based on external hadron beams. Compared to photon beams they provide a different biologic effect, stronger collimation and energy-depending Bragg peak at range end. To fully exploit the latter in treatment planning, accurate knowledge of tissues stopping power and patient positioning are required. Monte Carlo techniques (MC) are already used in hadrontherapy treatment planning, in this work they are used to evaluate the expected performance of a real-time monitoring system of secondary particles produced by beam-patient interaction, to assess treatment correctness. FLUKA is a MC simulation package, originally developed for particle physics, but already widely used for other applications in astro-particle and medical physics [Battistoni et al, HSSW proc.2006]. FLUKA simulations and ROOT-based tools are implemented to study the co-relation between activity projections and dose distribution. This work presents a general tool applied to the simulation of the INSIDE PET system. CASE STUDY: The INSIDE project (INnovative Solutions for In-beam DosimEtry in Hadrontherapy) aims at building a combined monitoring system featuring a dual-head SiPM-based PET scanner and a fiber tracker, the former to detect the decay signal of 15O and 11C isotopes created by beam-matter interaction, the latter to study prompt secondary particles (fig. 1). It will be placed at CNAO (National Centre of Oncological Adrontherapy, Pavia, Italy). The PET scanner is based on LSF scintillator elements coupled to 3.2 x 3.2 mm2 Silicon Photomultipliers, forming blocks of 16 x 16 elements. Two heads (10 x 20 cm2 ca. each) will be placed at 30 cm from the isocentre. EXPERIMENTAL: A test measurement has been performed at CNAO facility, irradiating PMMA phantoms with protons and carbon beams, employing both test and treatment beams. Periodic beam structure is 1s beam on (spill) + 4s beam off (intra-spill). Secondary particles have been detected with 5 PET channels at different positions. Each channel is made with a scintillator crystal (3.2 x 3.2 x 20 mm3 LYSO) coupled to a 3.2 x 3.2 mm2 SiPM by FBK-irst. The front-end is based on the 32 channel TOFPET chip [Rolo et al, J.Instr. 2013]. Data have been seamlessly acquired with both beam on and off, with typical rate of 1 kHz and 100 Hz (depending on irradiation time). Photopeaks are clearly visible in energy spectra. SIMULATION: The simulation is validated with the reproduction of beam test data. Preliminary simulation results of the full INSIDE PET scanner show that the acquisition during the 1s spills does not contributes to the data quality since the signal is dominated by pair production triggered by high energy photons rather than beta+ decays. On the other hand, the expected coincidences number with proton beams acquired during the 4 s intra-spill intervals is 3.1e5, which is sufficient for satisfactory activity image reconstruction, especially along the beam direction.